1. Field of the Invention
This invention relates to a clean diesel fuel and methods of producing a clean diesel fuel. In one aspect, this invention relates to a method of producing a diesel fuel which provides reduced emissions of oxides of nitrogen ("NO.sub.x ") and particulate matter. In all other aspect, this invention relates to a clean diesel fuel composition which is economical to produce, meets regulatory specifications, and has desirable characteristics including acceptable aromatics content and cetane number.
2. Description of the Related Art
Federal and state legislative bodies and agencies have issued a number of rules applicable to the production of clean diesel as an attempt to reduce emissions from heavy-duty vehicles of NO.sub.x, carbon monoxide, unburned hydrocarbons, and particulate matter. Diesel fuel properties given the most attention are cetane number, aromatics content, and sulfur content. Federal regulations, for instance, require vehicular diesel fuel sold beginning Oct. 1, 1993 to have a maximum sulfur content of 0.05 percent and a minimum cetane index of 40 or a maximum aromatics content of 35 percent.
Some states have issued more demanding requirements. For example, the California Air Resources Board ("CARB") has adopted Section 2282, Title 13, California Code of Regulations ("Section 2282") which limits the aromatic hydrocarbon content of diesel fuel sold or intended for sale as a motor vehicle fuel in California starting Oct. 1, 1993.
Section 2282 establishes a basic California statewide aromatic hydrocarbon limit for vehicular diesel fuel of 10 percent by volume with a less stringent 20 percent standard for small refiners and a temporary 20 percent standard for independent refiners.
Sections 2282(a)(1)(C) and 2282(g) allow diesel fuel producers and importers to comply with the regulation with a set of diesel fuel specifications of their choosing if they can demonstrate the alternative specifications result in emission benefits equivalent to the emission benefits resulting from the 10 percent aromatic hydrocarbon standard (or, in the case of small refiners, the 20 percent aromatic hydrocarbon standard).
Section 2282(g) identifies a test procedure for comparative testing of a prototype ("candidate") fuel and a reference fuel representative of a diesel fuel with 10 percent aromatic hydrocarbons (or 20 percent by volume for small refiners) involving back-to-back tests using a specified heavy-duty diesel engine and identifies the statistical methodology to be used in comparing the emissions of NO.sub.x, particulate matter, and the soluble organic fraction of the particulate matter resulting from the two fuels, and establishes a process for certifying diesel fuel formulations that satisfy the regulatory criteria.
Section 2282(g)(1) requires that an applicant for certification submit to the Executive Officer of CARB for approval a proposed test protocol which includes detailed information on the entity proposed to conduct the tests, the test procedures, analytical test data on the candidate and reference fuels, the quality control and quality assurance procedures, and identification of any statistical outlier tests to be used. The same section also provides procedures for applicants to submit a certification application which includes the approved test protocol, all of the test data, a copy of the complete test log, and a demonstration that the candidate fuel meets the requirements for certification.
If the Executive Officer of CARB finds that the candidate fuel has been properly tested and meets the performance criteria, an Executive Order certifying the diesel fuel formulation will be issued which assigns an identification name to the specific certified diesel fuel. The Order must specify that the certified diesel fuel formulation has the following specifications: (1) a sulfur content, total aromatic hydrocarbon content, polycyclic aromatic hydrocarbon content, and nitrogen content not exceeding that of the candidate fuel; (2) a cetane number not less than that of the candidate fuel; and (3) presence of all additives that were contained in the candidate fuel in a concentration not less than in the candidate fuel, except for an additive demonstrated by the applicant to have the sole effect of increasing cetane number.
The lower aromatics content, of ten percent (10%) or less, requirement is believed to have been established by CARB as an attempt to seek to reduce, even beyond the scope of Federal requirements, diesel engine NO.sub.x emissions and particulate matter emissions. Certain public reports issued by CARB have reflected CARB's apparent opinion, believed to have been expressed near the time of issuance of the California clean diesel regulatory requirements, that diesel powered heavy-duty vehicles contribute over 50 percent of the NO.sub.x emissions and over 84 percent of the particulate matter emissions from all motor vehicles in California.
Currently, many refiners produce diesel having an aromatics content in excess of 35 percent. Reducing the aromatics content of diesel from 35 percent or more to 10 percent or below requires significant capital investments for such refiners for new processing units, or process unit modifications, to reduce aromatics levels by extraction or by conversion to other compounds. In addition, each refiner's latitude to produce reformulated diesel varies with refinery configuration and the number of, and capacity of, process units and related process flexibility available in each refinery. Alternatively, certain refiners may have to buy diesel with 10 percent or less aromatics at relatively high prices to blend with their high aromatics refinery diesel products to meet regulated highway diesel aromatics specifications. Such purchases could adversely affect the economics of diesel production for a number of refiners.
In response to these CARB regulations, many California refiners and diesel importers need an economical and competitive diesel formula and method to produce diesel. To be competitive, this formula needs to contain more than 10 percent aromatics and a competitive cetane number, but must be certified by CARB.
For example, one published report, SAE Technical Paper 930728 authored by Manuch Nikanjam and entitled "Development of the First CARB Certified California Alternative Diesel Fuel", stated that a diesel fuel formulation was prepared for certification purposes, which had an aromatics level of 22.5 volume percent, and a cetane number of 53.4. Nikanjam reported that this formula, when tested, failed the NO.sub.x equivalency requirements although meeting the particulate matter and soluble organic fraction requirements, and thus would fail to obtain CARB certification.
Nikanjam stated that his "experience suggested aromatics would have to be at or below the 20% level" to meet CARB standards for diesel. Nikanjam concluded that "[s]uccessful certification of a 19% aromatics, 59 cetane number fuel and a 15% aromatics, 55 cetane number fuel will give a refinery the much needed flexibility of producing either a higher aromatics/higher cetane number fuel or a lower aromatics/lower cetane fuel."
Nikanjam's paper sets forth a predictive model for NO.sub.x as a function of cetane number and aromatics, as follows: NO.sub.x (g/bhp-h)=5.296-(0.0161)(Cetane)+(0.0281)(Aromatics)
Applying this model to a 20% aromatics diesel fuel, a cetane level of about 65 would be required for certification. Many refiners face formidable problems and high capital and production costs in achieving the relatively high cetane and low aromatics levels described by Nikanjam and required by his predictive formula to meet mandated NO.sub.x levels.
There is thus a need for alternative diesel fuels and a more economical process to produce diesel fuels which meet regulatory requirements.